Semi Truck Engine Hot Air Exhaust Vent Aero I.Q.

[Shepherd777]

Hello All -

I'm not ready to post pics of my whole aero semi project yet. But I would like to solicit input, thoughts, and constructive suggestions regarding the aerodynamics of the hot engine compartment air exhaust vents.

Of course, one would normally not have a body panel protruding away from the body with a sharp cut-off at the rear. You would blend this area into the body to maintain undisturbed flow. But in this case, the skirts behind the steering tires are rolled inwards, and hot 200+ degree air will vent from this area. (fan-on temp is 230 degrees F.)

We will fiberglass a small flange around the perimeter of the vent openings, and add grills.

Do you think the outside body air traveling over the vent will merge successfully with the hot underhood air exiting from the vent?

I obviously do, because I built it this way. We have been too busy with other stuff to model it and run it on CFD yet.

We could always add an F1 Fence as the Ferrari did, but I am unsure if this would be necessary. Tuft testing will confirm any and all hypothesis.

[slowmover]

Isn't this form of underhood air management being done on the aero Peterbilts and KW trucks, even lowly Freightliners the past 10-years or so?

[drmiller100]

interesting idea.

I have a few thoughts - first, I don't think the size is NEARLY big enough. Keep in mind you are taking 1/2 of the waste heat of the entire 450 horsepower engine, and thinking it will go through that little hole, after the air has gotten hot. I think you are a factor of 20 off.

Second, I'm worried about air coming back along outside the tire, and "stacking up" on the fender - you might find you need to tip the lower part of the fender "in" below your vent to get a vortex to keep the air outside of the fender.

Third, I think a MUCH more interesting thing is to look at the chin spoiler. A chin spoiler in front of or even with the radiator will keep the air out from under the truck giving the air somewhere to go.

[Shepherd777]

Originally Posted by slowmover

Isn't this form of underhood air management being done on the aero Peterbilts and KW trucks, even lowly Freightliners the past 10-years or so?

Yes, but my goal is not as much underhood air management, as it is reducing drag and saving fuel. Afterall, this IS the Ecomodder site.

The Pete 387 below has the little black side vents on each side of the hood by their red Peterbilt badge emblem. Air is also exhausted on the Pete and my truck at the rear top of the hood at the base of the windshield.

Originally Posted by drmiller100

interesting idea.

I have a few thoughts - first, I don't think the size is NEARLY big enough. Keep in mind you are taking 1/2 of the waste heat of the entire 450 horsepower engine, and thinking it will go through that little hole, after the air has gotten hot. I think you are a factor of 20 off.

Agreed, to an extent. Again look at the Pete 387 vents above. About the same size as my rear vents. We also have an air channel inside of the hood that directs the engine cooling fan air to that rear vent location. And we have an engine cooling fan that is far better than anyone else has. The very unique cooling fan that we use has the hot air enter from its center section, and it is expelled around its perimeter towards that channel along the inside perimeter of the body. But again, underhood air management was not the primary goal. Reducing vehicle exterior drag was.



Second, I'm worried about air coming back along outside the tire, and "stacking up" on the fender - you might find you need to tip the lower part of the fender "in" below your vent to get a vortex to keep the air outside of the fender.

I am having a hard time visualizing exactly what you are describing here. But that is exactly the type of aerodynamic concern I was soliciting. The forward edge of both skirts are rolled inwards towards the longitudinal axis several inches. We still may go with an F1 fence, as the Ferrari uses.

Third, I think a MUCH more interesting thing is to look at the chin spoiler. A chin spoiler in front of or even with the radiator will keep the air out from under the truck giving the air somewhere to go.

One is coming, we just wanted to get some miles in the winter without one first.

[drmiller100]

Hey, thanks for the great discussion!
On the silver truck you show, the little vents up high are just to let underhood heat out when you shut the truck off - they are up high so the hot air can get out from under the hood. The VAST majority of air goes out underneath the truck.

As an example, look at the height of the front bumper. It is MUCH lower then the frame rails - the front bumper pushes air around the truck giving the underhood air a place to exit out under the truck.

On the fender. Look at the front fender of the silver truck. Air is going to go around the front tire, and stack up on the inside of the fender just in front of the step.

The skirt on the ferriari is for visual mostly, and to let a bit of air out from under the front of the car. On a Ferrari, they are tubbed, so air doesn't have a great way to get out teh front of the car.

[drmiller100]

here is an idea to lower drag on a semi.

Put an adjustable fin on the top of the sleeper. Make it so it pushes air up more or less with electric actuators.

Put an automotive barometric pressure electronic sensor on the back of the sleeper.

For a given load, for a given trailer, for a given wind condition, you can adjust the fin up or down to minimize air pressure between the truck and the trailer.

Then put fins on each side of the sleeper up and down the sides - again adjustable.

[slowmover]

Yes, but my goal is not as much underhood air management, as it is reducing drag and saving fuel. Afterall, this IS the Ecomodder site.

I should have been more specific: faster ariflow through the radiator inlet would seem to be conducive to aero as one can make smaller the inlet, overall, if op-temps are good throughout the work range. This worked on our Jeep Cherokee for flow, and work by others on this site shows the range of "closure" possible.

Now that bs is aside, man, that is one cool fan! Any more pictures or description available?

I'm gonna head over to a truck specific site and link this up. There are a few users there who I know will be quite interested in this thread.

.

[Shepherd777]

Originally Posted by drmiller100

On the fender. Look at the front fender of the silver truck. Air is going to go around the front tire, and stack up on the inside of the fender just in front of the step.

Now were talking about what I want to talk about in this post. That is exactly why I designed this truck the way I did. So some of the hot air from underhood exits this vent and then blends with the air on the leading edge of the rolled-in skirt.

The skirt on the ferriari is for visual mostly, and to let a bit of air out from under the front of the car. On a Ferrari, they are tubbed, so air doesn't have a great way to get out teh front of the car.

If by the word "skirt" you are referring to the F1 fence, I disagree. The F1 fence is there to maintain the flow around the bumper and air dam and some air traveling over the wheel and tire. The rolled-in door panel is used to let a bit of air out from under the front of the car.

[Shepherd777]

Originally Posted by slowmover

I should have been more specific: faster ariflow through the radiator inlet would seem to be conducive to aero as one can make smaller the inlet, overall, if op-temps are good throughout the work range. This worked on our Jeep Cherokee for flow, and work by others on this site shows the range of "closure" possible.

Our next rig will have active aero shutters like the Chevy Cruze Eco. Right now, the inlet is too big for winter and too big too warm up fast when cold outside. But you are of course correct. Radiator airflow is directly proportional to fuel economy.

Now that bs is aside, man, that is one cool fan! Any more pictures or description available?

That fan is a product of one of our major sponsors, Horton Cooling.

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I can't say enough good things about them. Great folks to work with. Smart and savvy. Been in business over 60 years. They flew their engineers on their company BeechJet from Minnesota to my shop in Connecticut twice, to help me engineer it and set it up. I designed and fabbed all the aluminum tube and bracket frame-work you see in the pics.

By the way, you are looking at probably the only Class 8 rig in North America with a crankshaft-driven fan. Horton's computer-controlled Stratis viscous drive is sandwiched in between the two. And they built me a beautiful, billet, CNC piece that bolts to the crank and ties everything else together.

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Fans and Fan Drives - WindMaster® Revolution / High-efficiency, Hybrid-flow Fans | Horton

[drmiller100]

it looks like an old school radial fan from a VW bug from the 60's. Is that correct?

Those radial fans make surprising amount of pressure. What diameter is the fan?